The present invention relates to packings for use in towers or columns for bringing about intimate contact between two immiscible or partially miscible fluids, either a gas and a liquid, or two liquids.
Packed columns or towers are used in diffusional operations which include absorption, stripping, scrubbing, distillation and extraction. Essentially, the packed column is a vertical shell which is placed upon a foundation and is filled with packing materials which are supported on a screen of an open pattern which does not offer a high resistance to flow. The columns may vary in diameter from about 1 inch for small laboratory purposes to 15 feet for large industrial operations. Industrial columns may be up to about 100 feet in height. The pressure of the column ranges from high vacuum to a high pressure, depending on both the chemical and physical properties of the system.
There are three main classes of packings -- broken solids, grids and shaped packings.
The broken solids are the least expensive form and are typically irregularly shaped bodies of stone, rock, tile or coke. Although they frequently are a good corrosion-resistant material, they are not as satisfactory as shaped packings either in regard to liquid flow or in regard to offering an effective surface for transfer. With broken solids, it is also difficult to form a packing of uniform sizes to produce a bed of uniform characteristics with a desired voidage.
Grid packings are relatively easy to fabricate and are usually used in columns of squared section. They may be made from wood, plastics, carbon, or ceramics. The main problem with grid packings is in obtaining good liquid distribution because, at high liquid rates, the liquid tends to cascade from one grid to the next without being broken up into fine droplets which are desirable for high interfacial surface.
Shaped packings include the commonly used Raschig rings, Pall rings, Lessing rings and Berl saddles. Most of these packings are available in ceramics, metals, glass, plastics, carbon, and sometimes rubber. The rings are extruded packings, and the saddles require punch dies for their manufacture. The main disadvantage of shaped packings is their relatively higher cost, particularly when small sizes are used.